How to turn on the flash for the nikon d3000?

The flash part that pops up at the top of the camera

Take your camera out of auto or program.

What is happening is the camera senses that there's not enough light to take a shot without using a flash. To do so will cause blurring due to the necessary slow shutter speeds.

Place the camera in the Aperture, Shutter speed or manual mode. The cameras flash will stay tucked in and not interfere with your shots.

Many times whenever I've got an question about the Most Basic Camera Operation I Always refer to my Owners Manual because the Owners Manual Covers so much of the Basic Camera Operation.

Check out Page 58 of the Owners Manual
http://www.nikonusa.com/...000_EN.pdf